The present invention relates to communication between a subcutaneously implanted lead-receiver and external transmitter. More specifically, the present invention relates to a proximity sensing and feedback regulation system for the reliable generation and propagation of programmed pulse excitation signals to an implant from an external controller by use of inductive fields. These excitation signals are used for stimulating excitable tissue, such as a nerve bundle or muscle.
There is mounting scientific evidence that pulsed electrical stimulation of peripheral or cranial nerves has beneficial effects as adjunct therapy for clinical states such as partial complex epilepsy, generalized epilepsy, urinary urge incontinence, Alzheimer""s disease, inappropriate sinus tacycardia, neurogenic pain, depression, and refractory angina etc.
Nerve and muscle cells have membranes that are composed of lipids and proteins, and have unique properties of excitability such that an adequate disturbance of the cell""s resting potential can trigger a sudden change in the membrane conductance. Under resting conditions, the inside of the nerve cell (neuron) is approximately xe2x88x9290 mV relative to the outside. As shown in FIG. 1, a neuronal process can be divided into unit lengths, which can be represented in an electrical equivalent circuit. Each unit length of the process is a circuit with its own membrane resistance (rm), membrane capacitance (cm), and axonal resistance (ra).
A nerve cell can be excited by increasing the electrical charge within the nerve, thus increasing the membrane potential inside the nerve with respect to the surrounding extracellular fluid. This fundamental feature of the nervous system i.e., its ability to generate and conduct electrical impulses, can take the form of action potentials, which are defined as a single electrical impulse passing down an axon. This action potential (nerve impulse or spike) is an xe2x80x9call or nothingxe2x80x9d phenomenon, (shown schematically in FIG. 2), that is to say once the threshold stimulus intensity is reached, an action potential will be generated. As shown in the Figure, stimuli 1 and 2 are subthreshold, and do not induce a response. Stimulus 3 exceeds a threshold value and induces an action potential (AP) which will be propagated. The information in the nervous system is coded by frequency of firing rather than the size of the action potential. The threshold stimulus intensity is defined as that value at which the net inward current (which is largely determined by Sodium ions) is just greater than the net outward current (which is largely carried by Potassium ions), and is typically around xe2x88x9255 mV inside the nerve cell relative to the outside (critical firing threshold). If, however, the threshold is not reached, the graded depolarization will not generate an action potential and the signal will not be propagated along the axon.
As shown in FIG. 3, the resulting membrane voltage change will affect adjacent portions of the membrane, and in a nerve, that will propagate as a nerve impulse. In FIG. 3 the impulse is traveling from right to left. Immediately after the spike of the action potential there is a refractory period when the neuron is either unexcitable (absolute refractory period) or only activated to sub-maximal responses by supra-threshold stimuli (relative refractory period). The absolute refractory period occurs at the time of maximal Sodium channel inactivation while the relative refractory period occurs at a later time when most of the Sodium channels have returned to their resting state by the voltage activated Potassium current. The refractory period has two important implications for action potential generation and conduction. First, action potentials can be conducted only in one direction, away from the site of its generation, and secondly, they can be generated only up to certain limiting frequencies.
Most nerves in the human body are composed of thousands of fibers, of different sizes designated by groups A, B and C, which carry signals to and from the brain. For example, a major nerve such as the Vagus nerve may have approximately 100,000 fibers of the three different types each carrying signals. Each axon (fiber) of that nerve conducts only in one direction, in normal circumstances. The A and B fibers are myelinated (i.e., have a myelin sheath, constituting a substance largely composed of fat), whereas the C fibers are unmyelinated.
Components of nerve stimulation systems include electrodes next to the nerve bundle or wrapped around the nerve and a lead with conductor connected to a pulse generator. Functional electrical stimulation realizes the excitation of the nerve by directly injecting charges into the nerve via the electrodes. The electrical field, necessary for the charge transfer, is simply impressed via the wires of the electrodes.
The power supply can be implanted, as in a cardiac pacemaker, or alternatively, there may be an implanted lead-receiver and an external transmitter, with their respective coils being inductively coupled. The implantable lead-receiver can be miniaturized utilizing the currently available electronic technology. Alignment of the subcutaneous coil-receiver and external coil-transmitter is critical for effective electromagnetic coupling. Since the receiver and transmitter coils are coupled, the degree of coupling depends, in part, upon the physical spacing between the coils and how they are placed with respect to each other (orientation). Such issues do not normally occur when a direct electrical connection is made between the pulse source and the stimulating electrodes.
The system described in this invention provides improved means for proximity sensing to aid in the optimal placement of the external-transmitting coil. Furthermore, it provides the means for continuous feedback regulation of output pulses, as the two coils shift relative to each other, in the course of activities during the day or night during sleep.
It should be obvious to one skilled in the art that various sensor modalities such as Hall effect, ultrasonic, inductive, capacitive etc. can be considered for proximity sensing. The Hall effect sensor opens and closes a circuit electronically based on changes in magnetic flux between the sensor and a target. This sensor can provide rotation speed or position measurement. The ultrasonic sensor emits an ultrasonic pulse that reflects back from an object entering the sonic cone. The time of reflection of the signal (dependent in part on the ultrasonic reflectivity of materials) provides a measure of the distance of the sensor from an object. An inductive proximity sensor consists of a coil and ferrite arrangement, an oscillator and detector circuit and a solid-state output. The oscillator of the inductive proximity sensor creates a high frequency field radiating from a coil in front of the sensor. A metallic object can enter the field and have eddy currents induced and detected from its surface. The capacitive sensor detects the approach of a target near its leading surface and this results in an increase in the capacitance. The increased capacitance results in the increase of amplitude of an oscillatory signal which can be detected. A GMR sensor was used in this embodiment as it affords large separation between the two coils across various mediums. This sensor uses magnetic field strength and direction of the magnetic field to provide relatively accurate location measurement of a magnetic material or target. The size of the sensor and associated circuitry are also quite favorable in this case.
The prior art addresses proximity sensing but not with the methodology disclosed here, and not with an implanted device with passive circuitry. The Mann U.S. Pat. No. 5,545,191 discloses a transcutaneous coupling device having an implanted unit and an external unit using Velcro(copyright)) for attaching the external unit to the skin in a proper location for optimal electromagnetic coupling between the units.
U.S. Pat. No. 5,314,453 (Jeufter) is directed to position sensitive power transfer between an implanted power supply and a magnetic field sensing device. In this patent, a magnetically operated reed switch opens when the field from the magnet is remote or misarranged to prevent transmission of high frequency power by the transmitter antenna coil. Whereas the object of this patent is to either deliver or not deliver power to the implanted power source, the primary objective of the present invention is to regulate the output to the passive lead-receiver, thereby regulating the amount of current delivered to nerve for stimulation.
U.S. Pat. No. 5,811,893 (Soyck) applies a second sensor to react to the interference fields in order to maintain the signal through the feed-forward loop and preserving the functional state of the approximation switch even in the case of magnetic field interference.
U.S. Pat. No. 4,352,960 (Dormer) and U.S. Pat. No. 4,736,747 (Drake) merely disclose transcutaneous coupling by magnetically securing the subcutaneous and supercutaneous members.
U.S. Pat. No. 5,117,825 (Grevious) is directed to radio frequency coupling between an external programmable component and an implanted cardiac pacemaker for the purpose of programming the implanted device.
The present invention comprises a transcutaneous signal coupling method and apparatus for detecting the position of a passive, implanted electrode and transferring an electric excitation signal to the electrode from an external source. The apparatus of the invention includes an external unit containing a power source, a pulse generation circuit, a proximity sensing circuit, an inductive coil, and an implanted or internal unit. The internal unit contains a small magnet composed of materials, such as Samarium-Cobalt, that exhibit Giant Magneto-resistor characteristics, a passive receiving circuitry and an inductive coil. These Magneto-resistor sensors overcome a weakness in conventional resistor and Hall effect sensors with good sensing under relatively large separation between the sensor and the target magnet. The sensor of the invention measures the direction of the field applied from the magnet to the sensor within a specific range of field strength magnitude.
In accordance with a feature of this invention, the condition where the supercutaneous primary coil comes in optimal proximity and is located in parallel to the secondary subcutaneous coil, along its axis, can be recorded. An indicator provides measure of the proximity distance. The external unit has indicators for low battery, no output state, failure condition etc. These indicators are mounted in the remote external package. The intent of the present invention is to provide the user with detailed information concerning the state of the system while in use.
In another feature of this invention, as the axis or distance between the primary and secondary coils changes, the output of the external circuit provides a signal for correction, thereby providing feedback regulation of the current that is delivered to the implanted passive lead-receiver.
Various other features, objects and advantages of the invention will be made apparent from the following description taken together with the drawings.